Set screw for medical implants

ABSTRACT

A low profile and removable set screw for use in connecting one medical implant to another and for similar uses. The set screw has a base and a head that is breakable from the base in a breakaway region at a preselected torque, such that the head allows rotation of the screw and torquing of a tip of the base against one of the implants during installation. The screw includes a groove or notch at the breakaway region to trigger breaking. The screw also includes a pair of slots extending radially from an outer threaded surface of the base part way into the base. The slots intersect with the breakaway region, but not with each other leaving a center core in the base. A tool is provided for removing the base after the head is broken away. The tool includes a pair of spaced ears that are received in the slots and provide for rotation of the base by rotating the tool. A bore of larger width than the slots is centered on each slot at the breakaway region providing relief for metal adjacent to the bores during torquing and breaking.

BACKGROUND OF THE INVENTION

The present application is directed to a set screw for use inapplications where it is important for the head of the set screw tobreak away to leave a clean profile, but for the set screw to beremovable, especially set screws for use in conjunction with medicalimplants.

Over recent years, medical implants for support of and correction ofdefects in various portions of the body, especially the spine, haveimproved dramatically. It is desirable to continue to improve theseimplants, so that the implants provide greater strength and durabilityand so that the implants are effective for the life of the patient.Furthermore, it is very desirable to provide such an implant which islow profile and interferes as little as possible with tissuessurrounding the implant. Because the set screw which holds togethervarious parts of the implant, especially spinal implants, has often beena limiting factor with respect to such implants, current attention hasbeen directed to improving the set screw.

Set screws of the type used in medical implants have among otherproblems two somewhat conflicting requirements. It is important to beable to grasp the set screw and apply rotational force to it until it istorqued to a particular preselected amount. Because the torque appliedto such set screws is comparatively high, this normally requires aprotruding head with an external surface which can be grasped by adriving tool to set the screw at the preselected torque.

On the other hand, if the head sticks outwardly from the set screw afterinstallation, then this presents a problem, since the head willinterfere with surrounding tissue, such as muscle, ligament and thelike. Consequently, set screws have been developed recently that have abreak off head that is designed to break away from a base of the setscrew upon the application of the preselect torque.

While the break away head satisfies the two above and notedrequirements, it presents a problem of its own. That is, once the headbreaks away, it is then quite difficult to grasp the remaining set screwbase to remove it, should the implant need to be adjusted duringinstallation or should later work on the implant require removal of theset screw.

Therefore, there needs to be some mechanism provided to allow the setscrew to be removed. While the force or torque required to remove thebase of the set screw after it has been installed is less than theinstallation torque, it is normally only approximately 70% of thatrequired to install the set screw. Thus, a relatively large torque mustbe applied to the set screw base to remove it. Because it is imbedded inthe implant and because it is low profile, there is little surface tograsp to allow the surgeon to remove the set screw, once installed.

Applicant, in a manner described in prior patent applications ofapplicant, and others have tried to develop different ways toeffectively remove the set screw base once it has been broken away fromthe driving head. One of applicant's prior concepts was to provide aninternal bore in the base of the set screw which may be a smooth wallbore or which may include some structure for assisting in the grippingof the wall by a tool and then utilizing a easy out tool or the like totry to grasp the interior of the bore for removal. Such structure wasdesigned to try to provide sufficient torque to remove the base and insome cases functioned satisfactorily for this purpose, but in othercases did not. Because of the high torque applied to the base, it is notan easy process to remove the screw in this manner. In addition, theleverage radius that can be used on the interior wall is substantiallyless than can be applied to the exterior wall of the set screw.

Consequently, applicant foresaw a need for a set screw that would have abreak away head to allow a preselected torque to be applied to the setscrew and then have the head break away leaving a lower section andfurther that provided the surgeon with structure near the radialexterior of the screw base that could be accessed by a tool that wouldallow the surgeon to apply removal torque to remove the set screw withsubstantially greater grip and with substantially greater mechanicaladvantage as compared to use of a tool in a central bore.

SUMMARY OF THE INVENTION

A set screw is provided that is especially suited for use in medicalimplants and the like wherein it is desired to have a low profile setscrew preferably that is installed at a preselected torque and is thenremovable should removal be required. The set screw includes a threadedlower section or base and a breakoff head.

In particular, the base and the head are originally joined at a locationhaving a partial or circumferential notch or groove in the circumferenceof the screw between the head and bore, such that as torque isincreasingly applied to the head, the torque on the head reaches apreselected amount and the head breaks from the base at the groovegenerally in a plane perpendicular to an axis of rotation of the screw.

The base includes a threaded surface that is radially spaced from anaxis of rotation and that is received in a similarly and matinglythreaded bore in a first medical implant or the like. The bottom of thebase includes an axially projecting tip that may be a point, a ring,both or another similar structure for engaging a second implant so as tosecure the second implant relative to the first.

The head includes an outer grippable surface for gripping and applyingtorque and an internal bore to receive tools for grinding duringinstallation.

Located along the outer wall of the screw and extending partially alongthe base and partially along the head are a pair of non-intersecting orspaced slots. The slots extend radially part way into the screw and arepreferably positioned opposite or diagonally with respect to each other.The slots extend through the threads on the base. The slots provide forremoval of the base after the set screw is installed and the head brokenaway.

A tool is provided to remove the base utilizing the slots. The toolincludes a manipulative handle operably connected to a pair of earsadapted to be received in the base slots. The ears are spaced, sized andpositioned to be received in the slots radially inward of the threads sothat the tool can be inserted into the slots without engaging orinterfering with the threads of the first implant bore to allowcounterclockwise rotational force to be applied to the base through theears by rotation of the tool by a user and in this way remove the basefrom the implants.

Also associated with the slots is a circular bore that is of slightlygreater diameter than the side to side width of the slots, the bore ispreferably positioned to be centered on the notch or groove along whichbreakage between the head and base occurs. The bore provides relief atthe location of breakage. That is, normally when breakage between thehead and base occur, some of the metal adjacent to the slots may betwisted into the slot opening, thereby preventing easy access of thetool to the slot. With the bore, adjacent metal that is twisted isreceived in the portion of the bore outside the slot, thereby allowingthe tool to be easily received in the slot.

OBJECTS AND ADVANTAGES OF THE INVENTION

Therefore, the objects of the present invention are: to provide a setscrew for use in conjunction with medical implants and the like whereinthe set screw has a low profile after installation, provides forinstallation to a preselected torque and is removable from the implantafter installation; to provide such a set screw having a breakoff headand a region of reduced cross-section produced by a notch or groove thatis centered in a plane perpendicular to the axis of installation so asto trigger breakage at a preselected location; to provide such a setscrew having axially extending and radially penetrating slots thatextend along the outer surface of the screw and intersect with thelocation whereat the head breaks from the base, the slots beingpositioned and shaped to receive a tool to apply counterclockwiserotation to the base to remove the set screw from the implant; toprovide such a set screw including a relief region adjacent to the slotsto receive twisted metal upon breakage of the head from the base so asto allow full access of the slots to the tool; to provide such a screwthat has such slots that are positioned to provide a mechanicaladvantage to removal of the base over that provided by mechanisms andsystems that remove the base through internal structure; to provide atool having ears adapted to be received in the slots for applying aremoval torque; to provide such a tool wherein the ears include grippingcleats; to provide such a set screw in combination with medical implantsto secure a first implant to a second implant; and to provide such a setscrew, tool and overall implant system that is comparatively easy touse, easy to produce, and especially well adapted for the intended usagethereof.

Other objects and advantages of this invention will become apparent fromthe following description taken in conjunction with the accompanyingdrawings wherein are set forth, by way of illustration and example,certain embodiments of this invention.

The drawings constitute a part of this specification and includeexemplary embodiments of the present invention and illustrate variousobjects and features thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a removable set screw with a break offhead and a base in accordance with the present invention.

FIG. 2 is a front elevational view of the set screw.

FIG. 3 is a side elevational view of the set screw.

FIG. 4 is a cross sectional view of the set screw, taken along line 4--4of FIG. 3.

FIG. 5 is a fragmentary side elevational view of the set screw on areduced scale positioned in an implant with a head of the set screwbroken away from a lower section thereof and having portions removed toshow detail thereof.

FIG. 6 is an enlarged fragmentary top plan view of the set screw andimplant of FIG. 5 after breakaway of the set screw head.

FIG. 7 is a side elevational view of a tool to remove the set screw basefrom the implant.

FIG. 8 is a fragmentary side elevational view of the implant of FIG. 5with the set screw base being removed by the removal tool.

FIG. 9 is an enlarged and cross sectional view of the set screw base andtool, taken along line 9--9 of FIG. 8.

FIG. 10 is an enlarged fragmentary view of the removal tool, takenwithin circle 10 of FIG. 7.

FIG. 11 is an enlarged fragmentary front elevational view of the removaltool.

DETAILED DESCRIPTION OF THE INVENTION

As required, detailed embodiments of the present invention are disclosedherein; however, it is to be understood that the disclosed embodimentsare merely exemplary of the invention, which may be embodied in variousforms. Therefore, specific structural and functional details disclosedherein are not to be interpreted as limiting, but merely as a basis forthe claims and as a representative basis for teaching one skilled in theart to variously employ the present invention in virtually anyappropriately detailed structure.

The reference numeral 1 generally designates a set screw in accordancewith the present invention used in conjunction with a medical implantsystem 3 (FIG. 5) and having an associated removal tool 4 (FIG. 7).

The set screw 1 has an upper section or head 10 and a lower section orbase 11 originally joined together along a planar breakaway zone orregion 12. The set screw 1 is rotatable about a central axis A thatpasses longitudinally through the center of the screw 1.

The head 10 has an outer grippable surface 15, which in the illustratedembodiment includes six planar faces 16 joined together in a hexagonalshape, such that the faces 16 lie in planes that are parallel to but arespaced from the axis A.

The surface 15 is sized and shaped to receive a driving or installationtool (not shown) of any conventional type for torquing or clockwise (asviewed from a top 19 of the head 10) rotating the set screw 1. A bore 18extends from the top 19 of the head 10 partially through the head 10 andis coaxial with axis A. The bore 18 preferentially receives a nipplefrom a driving tool during insertion. Near the top of the bore 18 is arelief 21. A pair of tool receivers 22 and 23 extend between the bore 18and the surface 15 near the head top 19. The receivers 22 operablyreceive a portion of a driving tool during installation of the setscrew. A suitable driving tool for use with the set screw 1 of thepresent invention is illustrated in my co-pending application Ser. No.08/726,828 on TOOLS FOR USE IN INSTALLING OSTEOSYNTHESIS APPARATUSUTILIZING SET SCREW WITH BREAK-OFF HEAD, now U.S. Pat. No. 5,941,885,which is incorporated herein by reference.

The purpose of the head 10 is to allow a surgeon or other user to applya driving tool to grip the surface 15 and apply rotational force ortorque to the screw 1. The head surface 15 allows good leverage aboutthe axis A and mechanical advantage. As discussed in greater detailbelow, the head 10 breaks away from the base 11 at a preselected torque.

Although the illustrated surface 15 is hexagonal in cross section,surfaces of other shapes including non polyhedral shapes may be utilizedfor the grippable surface 15.

The base 11 has a radially outer surface 24 that is threaded and whichis coaxial with but spaced from the axis A.

The thread of the threaded surface 24 is sized and shaped to be receivedin an implant as described below. A bottom 26 of the base 11 has anaxially projecting tip 27 that provides frictional engagement andpreferably penetration into an object against which the screw 1 isoperably set. The tip 27 that extends outward parallel to the axis A.The tip 27 operably engages a structure against which the set screw 1 isset, also as described further below. The tip 27 of the illustratedembodiment includes a point 29 and sharp edged ring 30 that is radiallyspaced from the point 29. Although the point 29 and ring 30 are shown inthe illustrated embodiment, it is foreseen that other types ofprojecting tips may be used in accordance with the present invention.

Located between the head 10 and base 11 is the breakaway region 12. Thelocation 12 is in a plane that is typically perpendicular to the axis A.When the head 10 breaks from the base 11, as shown in FIG. 5, an uppersurface 31 is left on the base 11. The region 12 has a reduced crosssectional area perpendicular to the axis A, as compared to thesurrounding regions of the screw 1. Preferably, the breaking of the head10 from the bore 11 leaves the base upper surface 31 relatively smoothand free from burrs, although the metal will deform slightly sidewaysdue to twisting. The region 12 in the illustrated invention is producedby a circumferential groove 32 that has a center line 33 that lies in aplane perpendicular to the axis A. Although a circumferential uniformgroove 32 is used to trigger breakage in the present embodiment, it isforeseen that other structures such as a non-uniform notch may also beused for this purpose.

Extending radially inward from the head surface 15 and the base surface24 is a pair of spaced and non-intersecting slots 35 and 36. Each of theslots 35 and 36 is elongate with the greatest length being alignedparallel to the axis A. Each of the slots 35 and 36 is formed by millingor drilling and extends perpendicular to or radially inward toward theaxis A. Each slot 35 and 36 is positioned in opposed relationship ordiametrically relative to the other.

Each slot 35 and 36 extends substantially along the base outer surface24 and in the illustrated embodiment each slot 35 and 36 extends alongabout two thirds of the surface 24. The slots 35 and 36 extend throughthe breakaway region 12 and in the illustrated embodiment extend partlyalong the head surface 15. The depth of each slot 35 and 36 may varysomewhat, but must accommodate the removal tool 4. In the illustratedembodiment the slots 35 and 36 each extend approximately 60% of thedistance between the lowest part of the threaded surface 24 and the axisA. The width of each of the slots 35 and 36 in the illustratedembodiment is about one third of the diameter of the base 11, but it isforeseen that slots of other size may be used in accordance with theinvention, especially considering the size and shape of the tool 4 andthe overall diameter of the set screw 1.

Relief bores 40 and 41 are positioned to be centered over eachrespective slot 35 and 36 relative to the minor axes of the slots 35 and36, as shown in FIG. 3. The bores 40 and 41 are also centered on thebreakaway region 12. Each bore 40 and 41 is somewhat larger than theassociated slot 35 and 36 so as to form a relief area 43 and 44therebetween. The relief areas 43 and 44 receive deformed metal due totwisting and breaking at the breakaway region 12 to prevent such metalfrom entering and blocking the slots 35 and 36. The bores 43 and 44 arecylindrical and in the illustrated embodiment have the same depth as theslots 35 and 36.

The illustrated set screw removal tool 4 is T-shaped and includes anelongate shaft 50 with a handle 51 extending perpendicularly in oppositedirections from one end thereof. At an end opposite the handle 51 is aset screw engaging tool head 52. As is best seen in FIGS. 10 and 11, thetool head 52 includes a pair of axially projecting and slot receivingtabs, lugs or ears 54 and 55. The ears 54 and 55 are separated or spacedby a slot or channel 56. The ears 54 and 55 are sized and shaped to bereceived inward of the thread and within the slots 35 and 36 once thehead 10 has been broken from the base 11 and the channel 56 spans acenter core 57 of the set screw 11 between the slots 35 and 36. The ears54 and 55 adapted to be positioned or set in the slots 35 and 36, as isillustrated in FIGS. 8 and 9. The channel 56 is sufficiently large insize to receive the center core 57 of the set screw located between theslots 35 and 36. The ears 54 and 55 are also sized to not extendoutwardly into the thread of either the base 11 or mating thread of theimplant system 3.

Located near a lower end of each of the ears 54 and 55 is a cleat 58.The cleats 58 are positioned on the sides of the ears 54 and 55 thatengage the slots 35 and 36 when the tool is rotated counterclockwise toremove the base 11. As a substantial amount of torque is applied to thetool 4 to remove set screw base 11 once it has been secured to theimplant system 3, the cleats 58 serve to grip the sidewalls of the slots35 and 36 and reduce the likelihood of slippage. If one of the ears 54or 55 should break during removal of the base 11, the broken ear issimply removed and a new tool 4 is utilized for the purpose.

Shown in FIGS. 5, 6 and 8 is the set screw 1 in combination with themedical implant system 3. The medical implant system 3 that isillustrated includes a bone screw 60 and a rod 61. In the illustratedembodiment the bone screw 60 is implanted in a vertebral bone 62. Inconventional medical implant systems of the type illustrated herein,there are a wide range of parts that are utilized including hooks,connectors, bars, plates and numerous other elements that are allinterconnected together by means of set screws. The set screw 1 of thepresent invention may be used for interconnecting together any of thevarious elements.

In the illustrated embodiment, the bone screw includes a first bore 62that is sized and shaped to slideably receive the rod 61 and is oftenperpendicular to the central elongate axis of the bone screw 60. The rod61 is positioned in a first rod receiving bore 64 in the illustration.The bone screw 60 includes a second threaded bore 65 which intersectswith the first bore 64 and which is generally aligned to be coaxial withthe bone screw 60. The thread of the thread bore 65 is sized and shapedto receive the thread on the base outer surface 24 of the set screw 1.

The set screw 1 is, thus, first threaded into the threaded bore 65 untilthe projecting tip 19 engages the rod 61. Thereafter, clockwise rotationproducing an increasing torque is applied to the set screw 1 to set theset screw 1 both within the threaded bore 65 and against the rod 61. Asthe set screw 1 is rotated, the set screw point 29 and ring 30 engageand partially penetrate into the rod 61. As increasing torque is appliedto the set screw 1, the torque finally reaches a preselected torquewhere the head 10 breaks from the base 11, as is illustrated in FIG. 5.

The torque can be substantially varied according to the system and setscrew, but has been found that a torque of 90 inch pounds is highlyeffective when setting the set screw so as to prevent relative motionbetween the various elements of the system 3, including the bone screw60 and rod 61.

Subsequent to the head 10 breaking from the base 11, the upper surface31 of the base 11 is preferably beneath or substantially even with a top67 of the bone screw 63. In this manner the set screw 1 does not have aprojecting head 10 and is comparatively low profile.

Consequently, in operable use, the set screw base 11 secures a firstimplant such as the bone screw 60 to a second implant such as the rod 61by means of the set screw 1 being threadably held in the first implantbore 65 and set against the second implant or rod 61 by rotation andapplication of torque thereto.

Subsequent to the set screw base 11 being set, it is sometimes necessaryto remove the base 11. This occurs when the implant system 3 must beadjusted during installation or at a later time when original parts ofthe system must be removed for some reason. When this occurs, theremoval tool 4 is utilized. In particular, the removal tool ears 54 and55 are placed in the slots 35 and 36. Thereafter the user appliescounterclockwise rotation to the handle 51 to urge the ears 54 and 55against respective sides of the slots 35 and 36. Sufficient torque isapplied in this manner to unseat the base 11. The ears 54 and 55 aresufficiently radially inward so as not to interfere with the threads onthe interior of the bone screw threaded bore 65. This allows asubsequent set screw 1 to be utilized in the bore 65 to reset the bonescrew 60 relative to the rod 61.

While the torque required to unseat the set screw base 11 varies fromsystem to system and with the type of metal used, it has been found thatthe torque required to unseat the base 11 is often approximately 70% ofthe torque required to seat the screw.

It is foreseen that the set screw 1 of the present invention can beconstructed of many different types of materials. When the set screw 1is to be utilized for medical implants, the material construction shouldbe as compatible as possible with implantation and it has been foundthat stainless steel and titanium are typically preferred as materialsof construction for such uses.

Although the set screw of the present invention is described andillustrated in conjunction with mainly medical implants, as itadvantageously provides for many of the requirements peculiar to suchimplants as opposed to other types of set screws, it is foreseen thatthe set screw of the present invention may also be used with other typesof systems.

It is to be understood that while certain forms of the present inventionhave been illustrated and described herein, it is not to be limited tothe specific forms or arrangement of parts described and shown.

What is claimed and desired to be secured by Letters Patent is asfollows:
 1. A set screw for operably joining a first element to a secondelement comprising:a) a head having a surface for gripping, rotating andapplying torque to said set screw; b) a base initially joined to saidhead and having a threaded surface adapted to be received into athreaded bore of the first element; c) said head and said base beingjoined at a breakaway region adapted to break upon the application of apreselected torque such that said head separates from said base; and d)a pair of slots that do not pass through an axial center of rotation ofthe set screw base; said slots being located in and extending radiallyinward from said base threaded surface; said slots each intersect withsaid breakaway region; and said slots being adapted to receive a removaltool inward of said threaded surface subsequent to said head beingbroken from said base to allow application of torque to said base androtate said base counterclockwise.
 2. The set screw according to claim 1wherein:a) said slots are diametrically opposed and are separated by acenter core.
 3. The set screw according to claim 2 wherein:a) said slotsare elongate and have a major length parallel to an axis of rotation ofsaid set screw.
 4. The set screw according to claim 1 including:a) aregion of relief overlapping said break-away region and intersectingwith at least one of said slots; said region of relief adapted toreceive deformed portions of said base during setting of said baseagainst the second element such that said deformed sections do not blockaccess to said slots.
 5. The set screw according to claim 1 further incombination with a removal tool; said removal tool comprising:a) anelongate shaft; b) an operator grip for operably rotating said shaft;and c) a pair of ears projecting from said shaft; said ears being sizedand positioned so as to be receivable in respective slots of said baseafter said head is broken from said base such that removal torque may beoperably applied to said base through said ears by said removal tool. 6.The combination according to claim 5 wherein:a) said tool ears eachinclude a cleat adapted to engage a respective slot upon application ofremoval torque to reduce the likelihood of slippage of said ears.
 7. Aset screw according to claim 1 wherein:a) said slots are positioned,sized and shaped to be available for entry of a removal tool after saidhead is broken from said base and are not accessible by a removal toolprior to said head being broken from said base when said set screw baseis located within a threaded bore.
 8. A set screw according to claim 1in combination with the first and second elements wherein:a) said firstelement is an elongate member portion of a first medical implant; and b)said second element is a second medical implant having a first bore foroperably receiving said elongate member portion and a second threadedbore for receiving said set screw; said first and second boresintersecting.
 9. The combination according to claim 8 wherein:a) saidset screw is sized and shaped such that as said base engages and becomesset against said elongate member, said head breaks from said base suchthat said break-away region is entirely within said second threaded boreand thereby low profile.
 10. The combination according to claim 9wherein:a) said slots are not available for entry by a tool when saidset screw is in said threaded bore prior to said head breaking from saidbase and wherein said slots are available for entry by a removal toolafter said head breaks from said base.
 11. A set screw comprising:a) abase having a threaded surface adapted to be received in a threadedbore; b) a head originally joined to said base at a breakaway regionlocated by an exterior notch; said head being shaped and configured toreceive a driving tool for rotating and applying torque to said setscrew; said head breaking from said set screw at a preselected torque atsaid breakaway region; and c) a removal slot that extends radiallyinward from said threaded surface and is adapted to receive a removaltool, when said set screw is operably received in a threaded bore; saidslot being non-accessible by a removal tool when said head is joined tosaid base; and said slot is accessible by a removal tool after said headhas broken from said base.
 12. The set screw according to claim 11wherein:a) said slot is a first slot and including a second slot. 13.The set screw according to claim 12 wherein:a) said first and secondslots are diametrically opposed from each other and are spaced by acenter core.
 14. The set screw according to claim 11 wherein:a) saidslots are longer parallel to a rotational axis of said set screw thanwide and open into an upper surface of said base after said head breaksfrom said base.
 15. A medical implant system comprising:a) a firstimplant having a first bore and a second threaded bore intersecting withsaid first bore; b) a second implant operably received in said firstimplant first bore; c) a removable set screw for operably securing saidfirst implant to said second implant; said screw being threadablyreceivable in said first implant second threaded bore, abutted againstsaid second implant and thereafter torqued to a preselected torque; d)said set screw having:1) a base with an external threaded surface sizedand shaped to be threadably received in said first implant threadedbore, a bottom having a tip operably sized and shaped to frictionallyengage said screw implant and a top opposite said tip; 2) a headbreakably attached to said base top and adapted to receive aninstallation tool to drive said set screw base into said first implantthreaded bore; said head breaking from said base upon the application ofsaid preselected torque; subsequent to said head breaking from saidbase, said base being relatively low profile with respect to said firstimplant threaded bore; 3) said base also including a pair of slots; eachof said slots extending radially inward from said base threaded surfaceand intersecting with said base top; said slots being diametricallyopposed to each other and spaced by a center core of said base; whensaid base is in said first implant threaded bore, said slots are oftenaccessible to a removal tool said head is broken from said base and arenon-accessible to a removal tool when said head is attached to saidbore; and 4) a break initiating notch between said head and said base.16. The system according to claim 15 wherein:a) said set screw includesa pair of radially extending bores in said screw; each of said boresbeing positioned to intersect with a respective slot and with the basetop such that one of said bores operably provides relief adjacent eachrespective slot in association with the breaking away of said head. 17.The system according to claim 16 wherein:a) each of said slots iselongated parallel to the axis of rotation of said base.
 18. The systemaccording to claim 17 wherein:a) each of said bores is wider than saidrespective slots.
 19. The system according to claim 18 wherein:a) eachof said bores is centered in a respective slot so as to extend outwardon either side of a respective slot and is also centered on said notch.